The present invention relates to an electric power steering device that detects steering torque transmitted by a steering shaft by means of a torque sensor, and provides steering assistance power in response to the torque detected by means of an electric actuator.
Prior art electric power steering devices comprise a steering shaft which rotates by operation of a steering wheel, a torque sensor which is disposed around the circumference of the steering shaft and detects torque transmitted by means of the steering shaft, an electric actuator which generates steering assistance power in response to the torque detected, and a reduction gear mechanism which transmits the rotation of the actuator to the steering shaft. The reduction gear mechanism includes a driven gear arranged on the steering shaft and a driving gear that engages with the driven gear, and the torque sensor is disposed between the steering wheel and the driven gear.
Prior art electric power steering devices are further equipped with a column which covers a portion of the steering shaft between the steering wheel and the torque sensor, a sensor housing which covers the torque sensor, a gear housing which covers the reduction gear mechanism, and an end cover covering a portion of the steering shaft, which is further away than the driven gear from the steering wheel. The column is press fitted into an aperture formed in the sensor housing. The sensor housing, gear housing, and end cover are formed from cast aluminum in order to lightening the weight thereof. The gear housing is connected to the sensor housing with bolts on the side thereof which is away from the steering wheel. The end cover is connected to the gear housing with bolts on the side thereof which is away from the steering wheel.
The problems with the above prior art steering device are that there are a large number of parts, the manufacturing process needed to press fit the column into the sensor housing is complicated, and size of the device is enlarged due to the increase in the thickness of the walls of the sensor housing in which the column is press fitted. Furthermore, when the sensor housing, gear housing, and end cover are connected together with bolts, there are a large number of parts involved and the manufacturing process is complicated, because it requires parts such as bolts, the tapping, and the tightening of the bolts. In addition, when the sensor housing, gear housing, and end cover are formed from cast aluminum, there is a large amount of post-machining needed after casting because they greatly exceed their dimensional tolerances. Furthermore, bolts and the like are needed in order to fix a connection brackets to the vehicle body.
The object of the present invention is to provide an electric power steering device that can solve the aforementioned problems.
The present invention is an electric power steering device comprising a steering shaft which rotates by operation of a steering wheel; a torque sensor which detects torque transmitted by the steering shaft; an electric actuator for generating steering assistance power, which is driven in response to the torque detected; a reduction gear mechanism which transmits the rotation of the actuator to said steering shaft. The reducing gear mechanism has a driven gear provided on the steering shaft and a driving gear which engages with the driven gear. The torque sensor is disposed between the steering wheel and the driven gear. The electric power steering device further comprises a housing having a portion which covers the steering shaft between the steering wheel and the torque sensor, and a portion which covers the torque sensor. The housing is formed by plastically deforming a single metal pipe. The pipe is preferably, for example, a steel pipe.
According to the present invention, since the housing formed by plastically deforming a single metal pipe covers not only the steering shaft but also the torque sensor, the number of parts of the steering device can be reduced, the manufacturing process can be simplified because it is unnecessary to press fit a column into the sensor housing as with the prior art, the size of the device can be reduced and space can be saved because the walls of the housing do not have to be thickened for press fitting. Furthermore, since the housing is formed by plastically deforming a single metal pipe, the dimensional tolerances can be reduced as compared to casting, so that the amount of post-machining needed after the plastic deformation can be reduced. For example, the post-machining is needed only to the portions in which bearings are fit.
The present invention preferably comprises a gear housing which covers the driving gear and the driven gear, and is formed integrally with one end of the housing, the one end of which is further away than the other end from the steering wheel; and an end cover which covers a portion of the steering shaft, the portion of which is further away than the driven gear from the steering wheel, and the end cover is formed integrally with one end of the gear housing, the one end of which is further away than the other end from the steering wheel.
In order to further reduce the number of parts and further simplify the manufacturing process, it is preferred that the driven gear is covered with the housing, and a second housing is provided so that it covers a portion of the steering shaft, the portion of which is further away than the driven gear from the steering wheel, and also covers the driven gear, and the second housing is integrated with one end of the housing, the one end of which is further away than the other end from the steering wheel.
In the present invention, it is preferred that the inner diameter of the housing at one end, which is further away than the other end from the steering wheel, is larger than at the other end, the steering shaft is supported by the housing via bearings, and the bearings, the steering shaft united with the driven gear, and the torque sensor are inserted into the housing from one end of thereof, which is further away than the other end from the steering wheel.
In this manner, the assembling of the bearings, driven gear, steering shaft, and torque sensor into the housing can be performed from one end of the housing, the one end of which is further away than the other end from the steering wheel. In other words, because the assembling steps can be performed from one side of the housing, working efficiency can be increased.
In the present invention, it is preferable that a metal connecting member is attached to the vehicle body and welded to the housing. It is preferable that the metal connecting member is formed from, for example, steel.
In this manner, the number of parts can be reduced and the manufacturing process can be simplified, because bolts and the like are not needed to fix the connecting member to the housing.
The electric power steering device according to the present invention can provide a reduction in the number of parts, simplification in the manufacturing process, improving of working efficiency during assembly, and can save space.